Here's the challenge - Have the US acquire most of its OTL southwest (you can shave some off, as long as they have Alta California, Nuevo Mexico and Tejas) by direct conquest from Spain, not any Mexican state. Also, have US be an ally of rebels fighting for Mexican independence, and have the post-Spanish, independent Mexico consent (even if grudgingly) to US ownership of those old northern provinces of New Spain. Bonus points if the United States, especially its military forces, contributes a lot to Mexico achieving independence from Spain.